The present invention relates to MPEG digital video encoding and decoding (e.g., for digital television). In particular, the present invention relates to methods and apparatus for processing progressive I-slice refreshed Moving Pictures Expert Group (MPEG) data streams to facilitate “trick play” modes on a display device. Trick play modes include pause, fast forward, rewind, jump, still frame display, and the like.
In order to understand the concept of progressive I-slice refreshed MPEG data streams, it is necessary to understand some basics about the typical I-frame based MPEG data stream. MPEG gets much of its compression capability by sending only the changes between different video frames. A first type of frame, known as a prediction or “P” frame, contains an abridged set of data used by the decoder to predict a full frame from a previous “P” frame or from a previous complete frame (an intra-coded “I” frame) in the video stream. The stream merely carries “fine tuning” information to correct errors from an approximate prediction. An I-frame is compressed without motion prediction. Thus, a full video frame can be reconstructed from an I-frame without reference to any other frame. In this manner, errors in the prior frame predictions will be eliminated once an I-frame arrives and is decoded. Bi-directional “B” frames are like P-frames, except that the prediction is made not only from the previous I-frame or P-frame, but also from the next one. MPEG data streams encoded in this manner are referred to herein as “I-frame based MPEG data streams.” An I-frame based MPEG data stream may start with an optional Group-of-Pictures (GOP) header followed by an I-frame. The video frame can be reconstructed from the GOP without reference to other frame information.
In progressively refreshed streams, such as taught in commonly owned U.S. Pat. No. 5,057,916 to Krause et al. entitled “Method and Apparatus for Refreshing Motion Compensated Sequential Video Images,” sequential video images are refreshed one region at a time. A different region in each of the plurality of video images is communicated without compression during a refresh cycle. In this manner, an image area defined by the region is progressively refreshed by the non-compressed regions during the refresh cycle. Compression of video images may be controlled to prevent data contained in regions not yet refreshed during a current refresh cycle from corrupting data contained in regions that have been refreshed during the current refresh cycle.
When an MPEG encoder is used in the progressive refresh mode, the I-frames are broken into slices and distributed across the transmitted P-frames as I-slices. MPEG data streams encoded in this manner are referred to herein as “progressive I-slice refreshed MPEG data streams.” The spreading of I-slices over multiple P-frames equalizes the bit rate and does not present a problem when the stream is played back sequentially. However, when trying to implement trick play modes in a progressive I-slice refreshed MPEG data stream, the lack of I-frames requires either extremely large buffers for several pictures worth of data or the stream must be fully decoded and then re-encoded as an I-frame based stream resulting in a loss of quality. This additional decoding and re-encoding also requires extra hardware, thereby incurring extra cost in the display device (e.g., set-top terminal, digital video recorder, personal versatile recorder, and the like).
It would be advantageous to provide a method and apparatus for enabling trick play modes on progressive I-slice refreshed MPEG data streams and other types of progressively refreshed data streams. It would be further advantageous to provide such a scheme where the processing required for enabling trick play modes is minimized to allow for real time performance. It would be advantageous to provide for improved visual characteristics of video during trick play modes such as fast forward and rewind. It would be advantageous to provide such improvements regardless of whether the data stream is an I-frame based stream or a progressively I-sliced refreshed stream.
The methods and apparatus of the present invention provide the foregoing and other advantages.